Keep it simple!
Solid colors are usually best. No logos, no characters/bold patterns. Accessories such as hats, scarves, or big bags can add a little personality to your photos. Bare feet are always fun!
If planning for multiple people, try to create one big outfit- no one wearing the same thing, but pick pieces that compliment the one color palate. Try to avoid everyone wearing the same thing.
Due to the nature of my sessions, I encourage clients to choose a casual outfit & one semi-dressy.